1. Field of the Invention
The present invention relates to semiconductor integrated devices, and more particularly to a memory device with an improved passivation layer and the method for forming the passivation layer on the memory device.
2. Description of the Related Art
Typical semiconductor memory device fabrication forms semiconductor transistors, i.e. memory units, densely on a wafer and multiple metal layers with interconnects formed in electrical contact with the inside circuitry. In addition, a passivation layer is deposited over the entire top surface of the wafer to protect the underlying devices from contamination and moisture or mechanical damage.
The passivation layer is an insulating and protective layer that prevents mechanical and chemical damage during IC assembly and packaging for general semiconductor product. For flash memory, another purpose of the passivation layer is for hydrogen blocking, which is to prevent hydrogen from penetrating the interconnects. Because the floating gate of erasable programmable ROM stores negative charges as data storage, the hydrogen penetration into the interconnects will damage the underlying floating gate and affects the data retention and endurance of the memory device.
Conventionally, silicon nitride (SiN) is chosen as the material for passivation layer. U.S. Pat. No. 5,788,767 discloses a conventional passivation process that results in two separate layers: a silicon nitride layer atop a silicon-oxide layer which is deposited over the top metal of metal interconnects. Another conventional passivation layer is composed of two layers of silicon nitride. The reason to chose silicon nitride as the material for a passivation layer is because silicon nitride provides an impermeable barrier to moisture and mobile impurities, e.g. alkali metal ions. Generally, silicon nitride is deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) as a major layer of a passivation structure.
However, silicon nitride is not good enough for hydrogen blocking. For flash memory, data retention is seriously affected by the hydrogen penetration through the silicon nitride passivation layer.